1. Field of Invention
The present invention relates to fiber optic gain systems and in particular to fiber optic lasers and amplifiers including Vertical Cavity Surface Emitting Laser (VCSEL) optical pumps.
2. Background Art
Fiber optic lasers and amplifiers (hereinafter referred as fiber lasers and fiber amplifiers, respectively) are well known in the art for their excellent conversion efficiency, beam quality, small volume, light weight, and low cost. Primarily, fiber lasers and fiber amplifiers have an optical gain medium, typically comprising an optical fiber doped with optically active ions in a core region surrounded by a cladding region. For example, the core may be doped with selective rare-earth ions which upon absorbing optical power from an intense light source such as a high power optical pump, are excited to a higher energy state and when the excited ions return to their ground state they provide optical gain or amplification. The light generated in the doped fiber is reflected back into the cavity for resonant amplification.
Primary application of low power lasers and amplifiers for example, in 10-100 mW (milli-Watt) range is for amplifying optical signals in telecommunications. For other industrial and military applications of lasers that are very diverse, for example, cutting, welding, marking, etc. high output power is required. The output can be multimode or single mode and results in very high power densities in a focused beam. In addition the fiber laser or amplifier output may be generated to operate in continuous mode or pulsed with various pulse widths and repetition rates. In a recent non-patent literature publication entitled “110 W Fiber Laser”, presented in a Conference on Laser and Electro Optics (paper CPD11-1, at CLEO '99, 23-28 May 1999, Baltimore, Md.), Dominic et al., described a high power fiber optic laser using an Yb (Ytterbium) doped double-clad optical fiber specially designed to have a rectangular cross section cladding and a single mode core. In the configuration described therein, the double-clad fiber is pumped at both ends by polarization combining two laser diode bar packages, each package having an output power of 45 W, to achieve a total 180 W of optical pump power.
Most prior art fiber lasers and amplifiers focused on optimizing the fiber design and in particular, design of the fiber core, so as to couple pump radiation optimally. Advancements in core region designs made significant improvement in coupling pump light using different types of high power optical sources including edge emitting laser diode bars, Raman pumps, Erbium Doped Fiber Amplifier (EDFA) pumps and Vertical Cavity Surface Emitting Laser (VCSEL) particularly single VCSEL or a linear array of VCSEL, etc. Significant progress in attaining high power in fiber lasers and amplifiers may be attributed to advancements in new types of optical fibers particularly, a double-clad optical fiber. In a double-clad fiber, an inner cladding is used as a pump cavity, whereas the outer cavity prevents the pump radiation from leaking out. More specifically, pump radiation is coupled into the inner cladding and as it propagates down the fiber it propagates in and out of the core region but stays confined within the inner cladding at its boundary with the outer cladding.
In several prior art fiber lasers, inner cladding having in different shapes are constructed for optimally coupling the pump radiation to the core region. For example, in the U.S. Pat. No. 4,815,079 issued to Snitzer et al. on Mar. 21, 1989, a fiber laser and amplifier is constructed from a double clad fiber having a single mode core placed off-centered with respect to a multi-mode inner cladding and an outer cladding. The pump radiation is launched in the inner cladding layer. In addition, the fiber is configured to be slightly bent such that modes that would not ordinarily couple with the single mode core would couple pump power to the core to ensure efficient coupling of the pump power.
A different configuration for efficient coupling of pump power to a double-clad fiber is described in the U.S. Pat. Nos. 5,949,941, and 5,966,491 both issued to DiGiovanni on Sep. 7, 1999 and Oct. 12, 1999, respectively. In this design, an inner cladding region includes a stress inducing region around the core which is asymmetric, followed by a second cladding layer. The stress inducing region is for generating refractive index modulation to increase the pump radiation mode diversity to increase pump radiation propagation through the core region. A similar design is also described in another U.S. Pat. No. 6,477,307 issued to Tankala et al. on Nov. 5, 2002 having a inner cladding region with multiple sections that are designed to increase the amount of propagation of the pump radiation in the core region.
Output power in a fiber laser or amplifier is determined by input power from the pump source(s) as well as the proportion of the wavelength band which aligns with the absorption band linewidth of the active ions in the core region. In U.S. Pat. No. 7,593,435 issued to Gapontsev et al. on Sep. 9, 2009, a fiber laser capable of delivering 20 kW output power in a single mode beam is described. However this is a very complex arrangement where multiple single mode Er doped fiber amplifiers using Raman pumps, are used as pump source. The multiple fiber amplifiers are combined in single mode to multimode fiber combiners to generate a high power pump source. The narrow wavelength band closer to the emission wavelength, aligns accurately with the doped core fiber absorption line resulting in very efficient optical pumping due to high power, narrow linewidth and low photon defect.
As an alternative, edge emitting semiconductor lasers, a linear array of plurality of such lasers (or a laser bar), and extended emitter laser diodes (or arrays) emitting at wavelengths corresponding to absorption wavelength of doped ions in the fiber core are conventionally used as pump sources. One such pump configuration is described in U.S. Pat. No. 4,829,529 issued to Kafka on May 9, 1989. More specifically, a single mode fiber is embedded in a multi-mode cladding and an outer cladding layer to form a pump cavity such that the light from the pump source is confined within the pump cavity, by total internal reflection. The pump radiation to the core is coupled along the length of the fiber.
Another variation of pumping an inner cladding layer is described in the U.S. Pat. No. 6,801,550 issued to Snell et al. on Oct. 5, 2004. In this device a cladding layer in a double cladding fiber includes V-grooves to couple pump radiation efficiently to the core using multiple emitters placed along the length of the doped fiber. However, one disadvantage of this configuration is that the V-groove is designed integral to the cladding layer and works well for a specific pump wavelength.
While pumps using edge emitter lasers are useful for pumping doped core of fiber lasers and amplifiers, edge emitter lasers have a relatively broad linewidth and all the light emitted by the pump laser does not contribute towards exciting the doping ions which absorb only in a narrow wavelength band. Furthermore, edge emitter laser wavelength varies considerably with operating temperature resulting in misalignment in the pump wavelength and the absorption lines of the doping ions. Therefore, each edge emitting laser in a bar requires a temperature controller mechanism to stabilize respective operational wavelength.
In recent years, optical pumps for solid state lasers have been configured using VCSEL devices. VCSELs have very narrow linewidth and in addition their wavelength varies much less with temperature and drive current. The VCSEL chip can be configured with arrays of VCSEL devices in two dimensions which results in very high output power from a single array. In the U.S. Pat. No. 6,888,871 issued to Zhang et al. on May 3, 2005, an optical pump using VCSEL array is described for pumping a solid state laser. VCSELs and arrays of VCSELs are very compact and may be easily integrated with other devices for providing additional optical and control functions to the pump.
In other prior art patent publication for example, in the U.S. Pat. No. 7,295,375 issued to Jacobowitz et al. on Nov. 13, 2007, VCSEL arrays in combination with micro lenses and optical interconnects are described. A compact packaging for VCSEL arrays to a fiber cable is described in the U.S. Pat. No. 6,984,076 issued to Walker Jr. et al. on Jan. 10, 2006. However, each individual VCSEL is wire bonded to a contact pad and to the external connectors of the packaging. One major disadvantage of this approach is that for high optical power application, the wire bonding is prone to failure, thereby imposing a practical limitation on output power that can be obtained from such pumps.